Are you able to get Image Path correctly? I have a feeling you are not getting Image Path correctly. I have made minor change to your code. Let me know if that works for you. Incase it didn't let me know at what point it is failing.
if (pubPage != null && pubPage.Layout.Title == "ABCHome page Layout")
{
//Get the list
SPList HomepageImage = currentWeb.Lists["Homepage Images"];
//Get the list items
SPListItemCollection collListItems = HomepageImage.Items;
string strBannerImage = HomepageImage.Items[random.Next(0, collListitems.Count - 1)].Url;
//Select one of the list items
Random random = new Random();
bannerImage = "<img src='" + strBannerImage + "'>";
break;
}